1. Field of the Invention
The invention relates to methods of transparentizing fibrous sheets such as paper and to the transparentized fibrous products of the method.
2. Brief Description of the Prior Art
Transparent papers and like fibrous sheets are widely used in the packaging, printing and engineering fields. The purposes for using transparentized fibrous sheets such as paper vary from a need for reduced visual and reprint opacity to needs for chemical and physical resistance to degradation under certain conditions of use.
Engineering and reproduction applications of transparentized papers require certain physical and chemical characterstics such as good pencil and ink receptivity for tracing media, and a receptivity combined with frictional resistance which overcomes uneven penetration of coating solutions used in reprographic papers. When these papers are used in drafting, visual brightness is required and high UV translucency is necessary for making good reproductions. In practice, each grade of transparentized paper serves only a particular application. These papers are not interchangeable for each particular application. It is also desirable that a transparentized paper have good physical strength to resist handling, good permanence for record keeping, and chemical stability. Components applied to render the paper transparent should not migrate or volatilize, since that would result in a loss of transparency and contamination of interfield documents.
Paper is a fibrous material composed of a plurality of discrete fibers disposed in a sheet configuration with many voids between the fibers. The disposed fibers scatter incident light. The physical process of rendering paper transparent comprises filling the voids between the fibers with a material that has a refractive index identical or very close to that of the paper fibers. It should be noted that mineral fillers in papers affect the transparency because of their light absorption and light scattering properties and that papers without mineral fillers are preferred for transparentizing use.
Transparentizing paper is a widely established art and has been previously accomplished by impregnating the paper with mineral oils or waxes, polymeric thermoplastic resins such as polystyrenes, polybutenes, polypropenes, various rosin derivatives, and various polymers or copolymes of acrylics and styrene, with or without plasticizers. The application of such materials to the paper is done by either hot melt applications or by applying organic solvent solutions or aqueous emulsions to the paper. The processing equipment used for applying or treating fibrous material such as paper, varies with the type of application employed. For example, the transparentizing mixture may be applied by extrusion, tub, or roller applications and excess material may be removed by a doctoring off process using scrapers, wire, squeeze rollers, or size press. Where paper is employed for transparentizing, it sometimes is "wet packed" after treatment to improve the distrubtion of the transparentizing agents in the body of the paper and then after a certain time the "wet packed" paper is passed through a dryer to remove the solvent vehicle.
Where the transparentized paper is to be employed for drafting use, the transparentizing treatment may be followed by a surface application with resins and toothing agents such as particles of silica.
There are many disadvantages and problems associated with the prior art processes for transparentizing fibrous materials. High molecular weight resins would be desirable for transparentizing fibrous materials because of their physical strength, heat and water resistance, and absence of tendency of the resin to migrate within or out of the finished fibrous material. Such resins, however, can be applied to the fibrous material only with great difficulty since they tend to accumulate near or adjacent the surface of the fibrous material and when impregnation is accomplished the resin is generally unevenly distributed throughout the fibrous material. Thus, the translucency of the product made therefrom is low and is not uniform.
Low molecular weight liquid resins of low viscosity or mineral oils readily penetrate the fibrous material and produce a material of high translucency, but the finished surface would be soft and would not be pencil resistant. The surface would ghost upon erasure. Also, the transparentizing agents tend to migrate from the paper to interfiled documents lying adjacent thereto. Further, such low molecular weight resins or low viscosity materials are volatile and cannot be used where the transparentizing fibrous material is later subjected to elevated temperature processing equipment, such as xerography machines. The surface of such transparentized materials are often tacky to the touch and therefore are undesirable.
Polymeric transparentizing agents which contain polaric and non-polaric groups in the chain are wetting agents and will distribute more readily through the fibrous material. However, such agents form a transparentized fibrous material which is too soft to the touch. Further, such transparentizing agents are mobile and easily leach out in water. They are not set in the fibrous material and would not be suitable for the later application of a diazo-sensitizing solution because such coating would be allowed to deeply penetrate the paper.
The process for transparentizing paper disclosed in U.S. Pat. No. 3,813,261 comprises transparentizing fibrous material by impregnation with a mixture of polyol ethers, a methylol malamine and an acid catalyst to produce an improved translucent material. However, the effective polyol ethers are commercially unavailable due to discontinuation of their industrial manufacture.
Other transparentizing processes comprise polycondensation of polyols with polyisocyanates in situ in the paper. The resulting polyurethane impregnated sheets, however, have insufficient translucency for tracing applications and upon aging product yellow discoloration, even if so called non-yellowing polyisocyanates are used. Further, transparentizing processes with polyurethane forming condensation reactions in-situ generally require the presence of substantial concentrations of non-polaric solvents like xylene to prevent accumulation of residual water in the fibrous material as well as hydroxy groups of cellulosic fiber to interfere with the condensation reaction. The condensation reaction actually takes place in a solution phase and as the polyurethane product resins are still dissolved in the solvent, it is in reality not an "in-situ" process, but the resin sediments out of solution and on to the fibrous material only after evaporation of the solvent at elevated temperature in a separate converting step. It can be well assumed that the polycondensation reaction still continues during the solvent evaporation at elevated temperature. The resin thus is not intimately bound to the fibrous material nor homogeneously distributed to give a low transparentizing effect.
Conventionally, "in-situ" polymerized fibrous materials undergo heat, photo or electron beam polymerization immediately after impregnation in a continuing web pass. In such cases, extreme polymerization conditions are mandatory for economic reasons in order to shorten the reaction time. The polymerization can be considered a spontaneous one. Thus obtained transparentized sheets are more or less stratified materials and have a resin film on the surface which is detrimental to tracing applications, reduces receptivity for diazotype coating solutions, and is a cause for curl problems. If in such applications spontaneous polymerization is avoided through milder conditions, polymerization is incomplete and monomers or dimers remain in the composition. This makes the sheet useless for various applications. Spontaneous polymerization can also result in brittleness of the polymer, causing breaking of the resulting paper sheet, under the pressure of a hard pencil.
It is an objective of the invention to overcome the shortcomings of currently known transparentized fibrous sheets and to provide an improved transparentized fibrous material in sheet form which excels by degree and evenness of translucency, its surface qualities for pencil and ink take and erasure, receptivity of diazotype coatings and its physical endurance. The invention contemplates providing a novel composition for transparentizing fibrous material and the process of making it. We have found that certain tri or higher functional polymerizable monomers, when evenly dispersed within a fibrous sheet, without any excess left on its surface, and when slowly polymerized under controlled temperature conditions with an appropriate catalyst for thermo-polymerization, produce a three dimensional resin reinforced fibrous sheet which excels over currently available material in overall performance and physical endurance.
We have also found that a thorough dispersion of the monomeric material within the fibrous sheet can be best obtained by first applying the monomeric material in conventional manner then winding up the impregnated material in a tight roll, at room temperature, the keeping the thus "wet packed" roll for a certain period of time at a temperature at which no polymerization takes place.
We have further found that a retarded polymerization produces a higher quality transparent sheet than one obtained by spontaneous polymerization and we have found that we can perform such a retarded polymerization by exposing the tightly wound wet packed roll to an elevated temperature for an extended period of time, until polymerization is complete. While the process of the invention is not fully understood, we assume that the polymerization starts from the outside of the impregnated roll (which reaches polymerization temperature first). The heat generated from the exothermic polymerization reaction transmits toward the interior of the roll, thus propagating initiation of polymerization toward the center of the roll. Excessive temperatures which might cause spontaneous polymerization are avoided since a substantial part of the generated heat is absorbed by the fibrous material which is present at a ratio of between 14 and 5 to 1 over the resin forming material in the composition. The resulting transparent sheet has a surface which is matte to the eye and has a toothful feel which is beneficial for tracing purposes as well as for receptivity of diazotype coating preparations. Opacity values as low as 30% (Bausch & Lomb) are easily obtained.